1. Field of the Invention
The present invention relates to a process for preparing a fiber reinforced plastics (hereinafter referred to as FRP). More particularly, it relates to a process for preparing FRP product which has no filament cracking, layer peeling, or non-anistropy and has excellent fabricatability with the maximum fiber reinforcing effect and the plastic product.
2. Description of the Prior Art
FRP product is especially important among composite materials in view of light weight, high strength characteristics, and high elasticity characteristics.
FRP products have been prepared by a hand lay-up (HLU) method, a spray-up (SU) method, a preform matched melt die (Prf-MMD) method, a cold press (CP) method, a resin injection (RI) method, a sheet molding compound (SMC) method, a bulk molding compound (BMC) method, a vacuum bag method, a filament winding (FW) method and an autoclave (AC) method.
The filament winding method imparts remarkably excellent fiber reinforcing effect. In the other methods, the fiber is usually cut in short or the fiber is waved at crossing portions as a woven fabric and accordingly, the reinforcing effect resulting in excellent mechanical characteristics of the fiber is not satisfactorily attained. In accordance with the filament winding method, the continuous filaments under a wound condition are bonded in the fabrication. Therefore, the reinforcing characteristic of the filament is not deteriorated. In accordance with the filament winding method, the fiber density (Vf) can be remarkably high. This is one of the factors for high strength and high elasticity of FRP product.
The filament winding method is suitable for fabricating pipes or cylindrical or special products, however it is difficult to apply for fabricating a product having comprehensive configuration.
It has been known, as a process for preparing FRP product by the filament winding method, to produce the FRP product by winding a resin impregnated continuous filaments in the direction of 90 degree to the axis of a mandrel; cutting a semi-cured sheet on the mandrel to obtain a prepreg reinforced in one direction; and curing the prepreg by a vacuum bag method or an autoclave method. In this case, the filament layers are formed to give a desired thickness of the prepreg, however, there is no entanglement of the filaments between the layers and accordingly the layer peeling is disadvantageously caused. There is no entanglement of the filaments in one layer and accordingly, filament cracking is easily caused. Once, such defect is caused, the effect for preventing the growth of the defect is quite small.
On the other hand, the FRP products prepared by the filament winding method in the form of a cylindrical container or in the form of a prepreg laminated product have the remarkable anisotropic characteristics of strength and elasticity together with the effect of the reinforced filaments. Therefore, in the preparation of a pressure durable container by the filament winding method, the filaments are carefully wound in the various directions as the same direction and axial direction to reinforce it. On the other hand, the equal three directional reinforcing method has been considered for a prepreg lamination such as a plate.
The anisotropy of the prepreg laminated product in one direction has also been considered. As one example, a carbon fiber reinforced plastic prepared by the lamination in one direction has a strength in the filament direction of about 20 to 30 times the strength in the perpendicular direction. A carbon fiber reinforced plastic prepared by lamination in two directions of 0 degree and 90 degree has a strength of about 5 to 7 times the strength in the direction of 45 degree. A carbon fiber reinforced plastic prepared by the lamination in three directions 0 degrees, +60 degrees and -60 degrees has a strength substantially equal to the strength in the direction of 30 degrees. Thus, the one directional prepregs are reinforced in three directions to result in substantial isotropy. The product can be effectively used as parts or plates which should have uniform strength and elasticity.
The conventional laminated products obtained by arranging one directional prepregs in three directions have the aforementioned disadvantages of the layer peeling, the filament cracking and the low prevention of growth thereof and the other disadvantages caused by the layers of filaments in one direction. The laminated FRP products obtained by arranging one directional prepregs in three directions has a substantially isotropic property for a tensile stress but has anisotropic property for a bending stress. When the bending stress is applied, the maximum tensile strength and compression elasticity are given in the filament direction of the outer layer. In the FRP product, especially the FRP product having a thin thickness, there is a tendency for deformation around the axis of the filaments of the outer layer caused by a curing contraction or a thermal contraction of the resin as the impregnated matrix and accordingly, it has been difficult to form a precise flat or curved surface.
Moreover, the FRP prepared by the lamination of the one directional prepregs has disadvantageously inferior fabricatability. The fabrication of a product having a concavo-convex configuration or a large curvature has not been easy.